1. Field of the Invention
This application relates generally to sample-and-hold circuits. More specifically, this application relates to a sample-and-hold circuit that is capable of detecting and controlling strong incoming illumination in an image sensor.
2. Description of Related Art
Image sensing devices typically consist of an image sensor, generally an array of pixel circuits, as well as signal processing circuitry and any associated control or timing circuitry. Within the image sensor itself, charge is collected in a photoelectric conversion device of the pixel circuit as a result of the impingement of light.
One example of a pixel circuit is illustrated in FIG. 1. As shown in FIG. 1, a pixel circuit 100 includes a photoelectric conversion device 101 (for example, a photodiode), a floating diffusion FD, a transfer transistor 102, a reset transistor 103, an amplification transistor 104, and a selection transistor 105, and a vertical signal line 106. As illustrated, vertical signal line 106 is common to a plurality of pixel circuits within the same column. Alternatively, a vertical signal line may be shared among multiple columns. Gate electrodes of transfer transistor 102, reset transistor 103, and selection transistor 105 receive signals TRG, RST, and SEL, respectively. These signals may, for example, be provided by the control or timing circuitry.
While FIG. 1 illustrates a pixel circuit having four transistors in a particular configuration, the current disclosure is not so limited and may apply to a pixel circuit having fewer or more transistors as well as other elements, such as capacitors, resistors, and the like. Additionally, the current disclosure may be extended to configurations where one or more transistors are shared among multiple photoelectric conversion devices.
The voltage at signal line 106 is measured at two different times under the control of timing circuits and switches, which results in a reset signal (“P-phase value”) and light-exposed or data signal (“D-phase value”) of the pixel. This process is referred to as a correlated double sampling (CDS) method. The reset signal is then subtracted from the data signal to produce a value which is representative of an accumulated charge in the pixel, and thus the amount of light shining on the pixel. The accumulated charge is then converted to a digital value. Such a conversion typically requires several circuit components such as sample-and-hold (S/H) circuits, analog-to-digital converters (ADC), and timing and control circuits, with each circuit component serving a purpose in the conversion. For example, the purpose of the S/H circuit may be to sample the analog signals from different time phases of the photo diode operation, after which the analog signals may be converted to digital form by the ADC. A single-slope ADC is illustrated in FIG. 1, including a comparator 110, a digital counter 120, and a ramp reference voltage Vramp.
FIG. 2 illustrates a waveform and timing diagram for the different timing phases in acquiring the reset and data signals from a pixel, as well as an example of the voltage VSL during different phases. In FIG. 2, the solid line illustrates a VSL signal when the incoming illumination is at a “normal” level; that is, within the typical range of operation of the pixel under suitable exposure control. As illustrated, the voltage VSL is a result of the photodiode collecting negative charges when it is exposed to light; thus, the lower normal signal indicates a higher illumination than the upper normal signal in FIG. 2.
As illustrated by the solid lines in FIG. 2, VSL settles to a steady voltage after the pixel has been reset. Then, in the above example where a single-slope ADC is used, the ADC measures the voltage VSL beginning with the start of the “reset noise integration” period. During this measurement, Vramp begins at a high level and then decreases linearly as a function of time from this initial high level. Simultaneously, the digital counter starts counting from zero while monitoring the output of the comparator so as to stop counting when the comparator changes state. At this point, the stopped count value is a digital value corresponding to the reset signal of the pixel. The data signal value of the pixel is then measured in a similar fashion after the signal line VSL has once again settled; i.e., during the “data noise integration” period illustrated in FIG. 2. The difference between the data and reset values is then interpreted as the amount of illumination on the pixel.
However, if the illumination is the very strong, such as when a camera including the image sensor is pointing at the sun, this interpretation may be incorrect. This is due to two main factors. First, strong illumination may cause the photodiode to saturate, which results in charges leaking from one pixel to another. Some of this leaked charge is collected by the FD node in the neighboring pixels. This is sometimes referred to as a “blooming” effect where a bright spot in one part of an imaged scene spreads into a neighboring area of the image, causing a larger bright area in the image than in the actual scene. Second, strong illumination may cause a fraction of light on the photodiode to leak into the FD of the same pixel, which causes the floating diffusion to also act as a photodiode and generate charges in response to the leaked light. Both of these factors cause VSL to decrease with time as illustrated in the dashed and dot-dashed curves illustrated in FIG. 2. These factors are proportional to the strength of the illumination; thus, stronger illumination causes a steeper decrease in VSL.
The effect of strong illumination on VSL causes the difference between the data value and the reset value to decrease. Thus, an area with strong illumination may actually cause the output pixel value (data minus reset) to decrease, resulting in an output pixel that is interpreted as gray rather than white. When the input illumination is very strong, VSL may drop very quickly, as illustrated by the dot-dashed curve of FIG. 2. In this case, both the reset value and the data value are at the lowest possible level of the circuit operating range, and thus the difference between the data value and the reset value is zero. As a result, the output becomes black when there is very strong incoming illumination. This is referred to as the “black sun spot” problem because the resultant output image shows a black spot when the camera is directly pointing at the sun.
Thus, there exists a need for a sample-and-hold circuit that does not suffer from the black sun spot problem when the image sensor is subjected to strong illumination levels.